1. Field of the Invention
This invention relates to an apparatus for stabilization treatment of ferromagnetic metal powder, and more particularly it relates to an apparatus for stabilization treatment of ferromagnetic metal powder particularly composed mainly of iron.
2. Description of the Related Art
The recent development of magnetic recording is noteworthy and advances in magnetic powder used therefor has also contributed, to the above development. Further, in order to achieve high density magnetic recording, development of ferromagnetic metal powder having a high coersive force and high saturation magnetization has been advanced.
However, since this ferromagnetic metal powder has a very high chemical activity due to its fine particles, the powder, if subjected to no treatment, is at once oxidized upon its exposure to air to lose its specific feature as its intrinsic magnetic material. In addition to such problem, heat generation and ignition occur due to its sudden oxidation to raise a problem of its handling safety. Still another problem is raised with respect to corrosion resistance which is most important among practical specific features of magnetic tape (hereinafter abbreviated merely to "tape").
Thus, in order to assure the quality and handling safety of ferromagnetic metal powder and the corrosion resistance of tape, adhesion of organic substances onto the surface of ferromagnetic metal powder or the surface oxidation treatment of ferromagnetic metal powder have been carried out.
However, as to the surface treatment with organic substances, a certain effect upon the stability of the magnetic powder itself is observed, but at the time of coating of the resulting powder, the number of choices of surfactant, solvent, etc. used for dispersing the organic substances is limited, thereby making difficult dispersion of coating more difficult; hence a high quality tape not only could not have been obtained, but also the treatment has been uttely ineffective to provide corrosion resistance to a tape.
As compared with the above-mentioned process, the surface oxidation treatment process i.e. the oxidized coating-forming process using diluted oxygen as an oxidizing gas has currently been most broadly employed and is an effective process. As an apparatus for practicing the process, reactors of a stirring vessel type (Japanese patent application laid-open No. Sho 55-164001/1980), a fluidizing vessel type (U.S. Pat. No. 4,420,330), and a fixed vessel type (Japanese patent application laid-open No. Sho 57-19301/1982) have been known, but a problem has been raised with respect to the apparatus used for forming a uniform and dense oxidized coating to thereby produce powder having a good quality.
The reaction which proceeds by means of the reactor of stirring vessel type is directed to a process of dispersing ferromagnetic metal powder in an organic solvent with stirring and blowing an oxidizing gas into the resulting dispersion to thereby form an oxidized film on the ferromagnetic metal powder, but in order to uniformly disperse the ferromagnetic metal powder in the organic solvent, it is inevitably necessary to grind powder granules into primary particles or secondary particles. However, the surface energy of ferromagnetic metal powder is so large that when the powder is ground, an agglomerate occurs; hence the surface oxidation not only becomes uneven, but also the agglomerate does not disintegrate to smaller particle so that it has a bad influence upon the physical properties of tape. Further, since it is necessary to carry out agitation inside the reaction vessel, a stirrer is provided therein; thus this agitation causes collision of particles with one another, resulting in pulverization and further agglomeration to thereby deteriorate the physical properties.
The reactor of the fluidizing vessel type is directed to an apparatus wherein ferromagnetic metal powder is fluidized in a gas to form an oxidized film, but it is difficult to establish fluidizing conditions and it is necessary to carry out good fluidization for uniform coating; thus pulverization and agglomeration due to collision of particles with one another occur so that a non-uniform oxidized film is not only formed, but also the physical properties of ferromagnetic metal powder are deteriorated.
As to the reactor of the fixed vessel type, since ferromagnetic metal powder is not moved, pulverization due to collision of particles with one another does not occur, but non-uniform oxidation due to uneven flow of gas occurs and in an extreme case, reaction abruptly proceeds due to local oxidation heat-generation; thus there is a fear that the operation itself is impossible.
As described above, conventional apparatus has been insufficient as an apparatus for producing ferromagnetic metal powder having a good quality and a stabilized surface.